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Here is the reprint of an article in Koi USA on this subject:

Beware the Stagnation Zone in Your Pond
Gravel and Rock Form Dead Zone
by Joseph F. Cuny
It has been brought to my attention that some pond builders (both Koi
and water garden) apparently are not familiar with basic pond design. In
particular, I am referring to the use of gravel and rock inside the
pond. As anyone who has been a pond keeper for a few years knows, that
is a real no no. Possibly this practice is a direct carryover from the
aquarium 'under gravel' filters, but it is a wrong application of the
concept.
The problem is that the gravel and rock form a dead zone, more
appropriately called a stagnation zone. Whether the pond has fish in it
or only plants, there are all kinds of organic material produced in the
pond. This organic material gets trapped in the stagnation zone and
guess what happens? It stagnates! Since the water does not circulate
very well in this zone, the water is deficient in dissolved oxygen, and
the organics are processed anaerobically. In other words, the organic
material rots or putrefies.
The end result of such putrefaction is the production of noxious gases
and disease organisms. I do not know what effect these would have on
plants but they are deadly on fish and are very unaesthetic. In time
such a pond would smell like a cesspool! This is exactly what happens in
an aquarium if the under gravel filter is not cleaned regularly. Even
with oxygenated water flowing through such a filter, there are dead
spots where the water does not flow and these have to be cleaned,
usually with some type of vacuum or siphon system.
If it is necessary to put rocks on the bottom of a pond, possibly to
simulate a natural stream, they should be well separated and bedded in
mortar to allow flow around them and to eliminate pockets where debris
could accumulate. The amount of work necessary to properly imbed gravel
probably eliminates the use of gravel. If it is thought that the gravel
will function as a filter, it should be placed such that it can be
fairly easily cleaned. Despite claims made by various people, all
filters must be cleaned. The only possible exception is the trickling
filter type that is self cleaning, but in the process dumps the waste
into the following stage where it must be captured and disposed of.
I have seen gravel filters built into a sump in the bottom of a pond.
Such a filter is almost impossible to clean, and the resulting pond does
not provide a healthy environment for the fish. With such a filter in a
water garden, I would expect the same type of problems. I realize that
many people believe that the root system of the plants provides space
for the organisms that degrade the organic material, but this is not a
suitable filter except for those organics that are soluble. Even these,
when aerobically digested, result in material that falls to the bottom
where it joins the leaves, stems, decaying roots, and so forth and then
putrefies.

Joe Cuny is a founding editor and contributes to KOI USA since its
beginning, over 20 years ago.

This article first appeared in the March/April 1999 KOI USA.

Some food for thought..

Happy ponding,
Greg


"~ jan" wrote in message
s.com...
Yes, that's what I believe too. Though I believe his plan is to maintain
it
himself, he is a retired gent.... a healthy retired gent, but still, once
one gets in those upper years (you know, like above 40) one never knows
what will strike one down temporarily or slow one down permanently. This
gentleman is at least in his late 60s. ~ jan

On Thu, 06 Mar 2003 16:35:22 GMT, Theron wrote:

In the case of the Aquascape system, the nursery will make most of its
money from the sale of the weekly/monthly chemical additions and the
once a year cleaning of the rocks. A pond of this kind is a cash cow
for maintanance as long as the pond owner has it, and has the
installer maintain it.

Theron


See my ponds and filter design:
http://users.owt.com/jjspond/

~Keep 'em Wet!~
Tri-Cities WA Zone 7a
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